Friday, 28 October 2016

Eastern
Gorillas, Gorilla beringei,
are considered to belong to two subspecies, the Mountain Gorilla,
Gorilla beringei beringei,
and the Grauer's Gorilla or Eastern Lwland Gorilla, Gorilla beringei graueri.
The Mountain Gorilla is considered to be Critically Endangered under
the terms of theInternational Union for the Conservation of Nature's Red List of Threatened Species, and have been subject to extensive
conservation measures and regular monitoring since the 1970s, with
the effect that their population is currently thought to be slowly
increasing. The Graurer's Gorilla, in contrast, is currently
classified as Endangered under the terms of the International Union
for the Conservation of Nature's Red List of Threatened Species, but
no organized survey of the population of this species has been
carried out since the 1990s, largely due to political instability and
conflict in the east of the Democratic Republic of Congo where the
majority of these Gorillas are found.

The only previous attempt at surveying the population of the Graur's
Gorrila took place in 1994-95, when conservationists surveyed the
population in the Kahuzi-Biega National Park, thought to be home to
about 86% of the species' population, coming up with a figure of 16
900 individuals, on top of which it is thought there were a further
1000-2000 individuals in the Tayna-Usala region. Since this time the
civil war in Rwanda in 1994 led to hundreds of thousands of refugees
to flee into eastern Democratic Republic of Congo, destabilizing the
area and helping to trigger the Congolese civil war of 1996-2003
which killed an estimated five million people, and destroyed much of
what little infrastructure existed in the east of the country. This
situation was further exasperated by refugees and militia groups
fleeing conflicts in South Sudan and Uganda, and by military
incursions by several neighbouring countries seeking access to the
area's rich reserves of gold and coltan minerals.

This has led to a situation where much of the east of the Democratic
Republic of Congo was in the hands of a variety of militia groups,
many of which also control mining encampments, and are at least in
part dependent on bushmeat (hunted game) for sustenance. Gorillas are
often considered a valuable target by bushmeat hunters, as their
large size means they provide a large amount of meat for a single
kill, and their ground-dwelling habit makes them easier to track than
the semi-arborial Chimpanzees. Against this background the Congolese
parks authorities have struggled to re-assert their authority in the
region, with more than 150 rangers from the Institut Congolais pour
la Conservation de la Nature having been killed in action over the
past 20 years.

Plumptre et al. collected data from foot surveys made by park
ranger and community ecogaurds (who operate on community run
reserves) between 2011 and 2015. They were able to collect data from
the Itebero-Lulingu and Nzovu sectors of the Kahuzi-Biega National
Park, which were surveyed in 1994-95, plus the Kasese sector of the
Kahuzi-Biega National Park, the Concession Forestière Communautaire
des Banisamasi and the Tayna and Kisimba-Ikobo reserves, which were
not.

From this data they were able to produce estimates of the population
of Grauer's Gorillas in each of these areas, with the largest
surviving population found in the Itebero-Lulingu sector of the
Kahuzi-Biega National Park, where there are now an estimated 655
remaining Grauer's Gorillas, compared to an estimated 5635 in
1994-95. This was followed by the Tayna and Kisimba-Ikobo reserves
(not surveyed in 1994-95) with an estimated combined population of
541 Gorillas. After this came the Nzovu sector of the Kahuzi-Biega
National Park, with an estimated 258 Gorillas, compared to an
estimated 1188 in 1994-95, then the Kasese sector of the Kahuzi-Biega
National Park with an estimated 183 Gorillas and the Concession
Forestière Communautaire des Banisamasi, with an estimated 24
Gorillas.

Map of the probability of presence for Grauer's gorilla. Occupancy
probability was mapped using three covariables (tree cover, altitude
and distance to recent deforestation) and a spatial autocorrelation
(left panel). The probability threshold maximising the TSS was 0.35.
This threshold was used to derive a gorilla distribution range (upper
right panel) for which presence probabilities were greater than this
value (grey = less than 0.35). Plumptre et al. (2016).

This gives a total estimated figure of 1661 surviving Grauer's
Gorrillas in 2011-15, compared to the figure of 17 900-18 900 for
1994-95. This implies that the Grauer's Gorilla has lost 86.6% of its
population in the Itebero-Lulingu and Nzovu sectors of the
Kahuzi-Biega National Park in the two decades from 1995 to 2015.
Gorillas can live for about 40 years, but a generation is generally
taken as being about 20 years, which means that this loss has
occurred in a single generation. One of the International Union for
the Conservation of Nature's criteria for being included on the Red
List of Threatened Species under the category Critically Endangered
is the loss 80% or more of a species population within three
generations. As the Grauer's Gorilla is known to have lost more than
80 of its population in a single generation across much of its range,
and is likely to have declined by a similar amount in the other,
previously unsurveyed areas, Plumptre et al. recommend that
this species be included on the Red List as Critically Endangered.

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About Me

Studied Palaeobiology & Evolution at the University of Portsmouth, Geosciences via the Open University & Ecology and Conservation at Christchurch University, Canterbury.
Have worked in wildlife based tourism, mineral exploration, development, conservation, education & environmental chemistry. Occasionally write articles for papers and magazines.

This Blog would be impossible without the work of countless scientists (and others) throughout the world. Where possible I do my best to credit them, but there will always be many more who remain unmentioned; this does not imply I am ungrateful for their contributions. Any errors or inaccuracies are, of course, my own.